With growing concerns about global environmental issues and energy issues, a solar cell has been paid attention as an energy generating means. Such energy is clean with no concerns about drying up. When a solar cell is used in outdoor environment such as on the roof of a building, it is generally used in the form of a solar cell module.
The aforementioned solar cell module is generally produced according to the following procedures. First, a crystalline solar cell element (hereinafter also referred to as the power generating element or cell) formed from polycrystalline silicon, monocrystalline silicon or the like, or a thin film solar cell element obtained by forming an ultra-thin (several micrometers) film made of amorphous silicon or crystalline silicon onto a substrate of glass or the like, is manufactured. Next, in order to obtain a crystalline solar cell module, a protective sheet for a solar cell module (surface protective sheet), an encapsulating material sheet for solar cell, a crystalline solar cell element, an encapsulating material sheet for solar cell and a protective sheet for a solar cell module (back surface protective sheet) are laminated in this order. On the other hand, in order to obtain a thin film solar cell module, a thin film solar cell element, an encapsulating material sheet for solar cell and a protective sheet for a solar cell module (back surface protective sheet) are laminated in this order. Thereafter, a solar cell module is manufactured through a lamination method in which the laminated material is absorbed under vacuum and pressed with heating. Solar cell modules manufactured in this manner are weather-resistant and thus are suitable for use in outdoor environment such as on the roof of a building.
As an encapsulating film material for a solar cell, a film made of an ethylene/vinyl acetate (EVA) copolymer has been widely used because it is excellent in transparency, flexibility and adhesiveness. For example, Patent Document 1 discloses an encapsulating film excellent in both adhesiveness and film-forming properties consisting of a crosslinking agent and an EVA composition containing trimellitate. However, when the EVA composition is used as a constituent material of an encapsulating material for solar cell, there is the risk of possibly affecting the solar cell element by the component such as acetic acid gas and other unwanted gas generated by decomposition of EVA.
On the other hand, there has been proposed the use of a polyolefin based material, particularly an ethylene based material, as an encapsulating film material, because it is also excellent in insulation properties (for example, see Patent Document 2).
Meanwhile, there has also been proposed a resin composition for an encapsulating material for solar cell using an ethylene/α-olefin copolymer excellent in a balance between rigidity and crosslinking properties, and extrusion moldability (for example, see Patent Document 3).
Also, with the recent popularization of solar power generation, solar cell power generation systems have increased in size. In general, in solar cell power generation systems, several to several tens of solar cell modules are connected in series, and in order to decrease transmission loss, and so forth, there is a movement to raise system voltages. For example, small-sized household systems have been operated at 50 V to 500 V, and large-sized systems called mega solar systems have been operated at 600 V to 1000 V. For the outer frames of solar cell modules, aluminum frames and the like are used to retain strength, and so forth, and from the viewpoints of safety, the aluminum frames are often grounded. As a result, between a frame and a solar cell element, and between glass having low electrical resistance (glass which is disposed on the surface of a solar cell module) and a solar cell element, a potential difference is generated.
For example, in case of a solar cell array having a system voltage of 600 V to 1000 V, in a module having the maximum voltage, a potential difference between a frame and a solar cell element becomes the same as the system voltage, that is, 600 V to 1000 V. Also, through the frame, a high voltage is generated even between glass and a solar cell element. In other words, under a situation where photoelectric conversion is being performed, in modules connected in series, potential differences between solar cell elements and frames and potential differences between the solar cell elements and glass increase sequentially from the ground side, and at a place having the largest potential difference, the potential difference in the system voltage is almost maintained. There is a report that due to that usage condition, in a solar cell module having a crystalline power generation element, a potential induced degradation (PID) phenomenon happens such that output power decreases considerably and property degradation occurs.